1. A means has been devised of estimating numerically the relative virulence of Staphylococcus aureus strains, from computer stored bacteriophage typing patterns plus the categories of infections caused. A high ratio of serious infections to simple "colonization" results in a high "Index of Infection Potential." 2. An analysis of the carriage of S. aureus in selected patients over a long time interval has been completed. 3. A unique hospital-wide epidemic of Staphylococcus aureus infections was documented (Walter Reed Hospital). The outbreak was notable for the wide dissemination of disease-causing strains among nurses, surgeons and patients, the morbidity and mortality rate, and the response to increased hand washing, glove use, and reduction in catheter use, without isolation practices.